BH250-99a
Title
Subject
Description
Minor Minerals: orthoclase, chlorite, sericite (secondary), oxides
The outcrop from which this rock sample was collected is a well-known geological site frequently visited by geologists across the Midwestern United States. It exposes a striking contact between two distinct rock types: a light-colored felsic (BH250-99a) rock and a darker mafic to intermediate (BH250-99) rock. One of the most frequently debated questions at the site is the relative age of the two units—which rock is older, and which is younger?
What makes this outcrop especially intriguing is that the answer isn’t immediately clear. In some parts of the exposure, the felsic rock appears to be older, with the darker rock intruding into or surrounding it. Elsewhere, the mafic-intermediate rock seems to be the earlier phase, with felsic material cutting through it or forming enclaves. This ambiguity has led many to conclude that the two magmas were emplaced nearly simultaneously, undergoing mingling and partial mixing before fully solidifying.
Chemical and petrographic analysis confirms that the felsic rock is a granite, while the darker rock has an andesitic composition. Thin section observations reveal fascinating mineralogical relationships, including quartz crystals mantled by amphibole—evidence of complex crystallization dynamics during magma interaction.
This outcrop provides an excellent teaching example of magma mingling, hybridization, and the challenges of deciphering igneous emplacement history. It continues to be a compelling stop for field-based discussions on igneous petrology and geologic interpretation.
Coverage
GPS location: 44°49'41.87"N, 95°33'38.20"W